The present invention relates to an electrochromic display and, more particularly, to a construction of an electrochromic display and a driving method for the same.
There are well known driving methods for an electrochromic display (referred to "ECD" hereinbelow) comprising a pair of substrates, a display electrode and a counter electrode each including an electrochromic material, and an electrolyte filled between the substrates. The driving method is classfied into three groups comprising a potentiostatic driving, a constant current driving, and a constant voltage driving method.
With respect to the ECD comprising the electrochromic material as a reactive material, the following problems exist:
(1) A balance voltage and an overvoltage of the electrochromic material are varied in accordance with a shade of color of the same. With an increase in the coloration intensity of the electrochromic material, the balance voltage is lowered and then the overvoltage is also decreased. Even if a constant voltage is applied to the ECD, current flowing through the ECD is varied in accordance with the shade of color of the electrochromic material. The required shade of color of the display electrode can not be obtained.
(2) There is a tendency that the counter electrode will be restored to its bleached state when the EC material on the counter electrode is colored and driving is halted for a long period. Although the ECD is said to have a memory effect, the colored state will, in effect, be debilitated little by little over a period of time. Should the driving conditions be proper, it will become more difficult to conduct current for the above mentioned reasons. The electro-optical performances of the ECD, therefore, will degrade in spite of non-degradation of the EC material itself.